Static converters of this kind are preferably used for system applications in the high power range of typically greater than 10 MVA.
One area of use is high-voltage DC transmission systems, wherein electrical power is transmitted over long line lengths using DC at high voltage of above 100 kV, the ends of the line lengths being connected to power converter stations with static converters of the type cited at the outset.
A further area of use is static power-factor correction systems in which static converters of the type cited at the outset improve the voltage quality and stability in transmission systems, allow the system connection of energy sources which provide a fluctuating supply, and compensate for reactive powers and flicker in industrial installations.
Another area of use for such converters is substations for the supply of traction to rail vehicles, which form the interfaces between a public high-voltage system, subsequently called national system, and the high-voltage system for supplying power to railways, subsequently called railway system. Whereas the railway systems are of single-phase design and have a frequency of 16.7 Hz or 25 Hz, the powering national systems are of three-phase design and have a frequency of 50 Hz or 60 Hz. To couple these two different systems, the converters of the type cited at the outset are used as static frequency converters.
A static converter of the type cited at the outset has a plurality of module branches which—depending on converter type—can be interconnected in a B6 bridge circuit or else in a delta circuit. In this arrangement, a plurality of submodules are electrically series-connected in each module branch. The voltage per submodule is typically approximately 1 to 3 kV; the voltage of a module branch may—depending on application—be in the range from approximately 10 kV to several 100 kV. Each submodule is in the form of a half-bridge or full-bridge circuit and has an associated capacitor as energy store and also power semiconductors as switching elements. The power semiconductors can connect the voltage of the capacitor to their power terminals with one or two polarities and also zero. A regulatory system actuates the submodules such that the series connection of the voltages to the power terminals of the two-pole submodules sets the currently desired total voltage of a module branch. The voltage tapped off between two module branches of a phase module can be set almost arbitrarily so as to build up the almost sinusoidal voltage. Static converters of this kind are referred to as modular multilevel power converters. A single-phase power converter with two phase modules and a three-phase power converter with three phase modules are known from “A New Single Phase AC/AC-Multilevel Converter For Traction Vehicles Operating On AC Line Voltage”, for example, published by M. Glinka, R. Marquardt, at the 2003 EPE Conference in Toulouse. Similarly, it is possible to implement other multiphase power converters.
Besides the excellent electrical properties, converters of this kind are distinguished by their modular design. They can be used very versatilely and cost-efficiently and have a high level of redundancy and availability. Installations in the areas of use listed at the outset can be designed in a very space-saving manner by using such converters.
“New Concept for High Voltage—Modular Multilevel Converter”, published by R. Marquardt, A. Lesnicar at the 2004 PESC Conference in Aachen, Germany, suggests a method for starting up such a converter which is intended to be used to charge the capacitors from a zero-voltage state to operating voltage. Charging the capacitors of a phase module requires an auxiliary voltage source with only a relatively low output voltage. In this case, the power semiconductors of a particular number of submodules in a module branch are connected such that the capacitors of these submodules are in the current path and are accordingly charged, whereas the capacitors of the other submodules are connected such that they are not in the current path. When a capacitor of the module branch has reached its operating voltage, appropriate switching selects the further capacitors. In this way, all capacitors in the module branches are charged to operating voltage in succession.
One particular feature of the submodules may be that the auxiliary voltage supply of the power semiconductors comes from the capacitor voltage, and said power semiconductors can therefore become active only from a particular minimum voltage onwards. When the converter is started up, it is therefore necessary to charge the capacitors of the submodules to the minimum voltage so that the converter can be connected to the supplying system or to a load. This process is called precharging. The method described above cannot be applied in this case, since the submodules cannot be actuated without an auxiliary voltage.
One opportunity for precharging is to connect the converter directly to the supplying system. A drawback in this context is that when the transformer between the system and the current converter is switched on, a greatly increased switch-on current can arise if the phase angle of the system voltage is unfavorable, because the iron core is driven to saturation. The associated reduction in the inductive reactance means that there is a brief flow of very large currents, called inrush currents. These inrush currents overlap the precharge currents, which can result in undesirably high loading on the network. Although switched precharge resistors can alleviate this problem, some power converter configurations do not allow the required minimum voltage for the capacitors to be achieved.